As the demand for high-speed telecommunications systems increases, more signal bandwidth is required for use therein. While attempting to augment the available bandwidth of channels within a given transmission frequency range (e.g., 2.5 to five gigahertz range), the combined use of amplitude and phase modulation such as may be found with orthogonal frequency division multiplex (OFDM) signals, has proven difficult to effectively implement. More specifically, the delivery of improved spectral efficiency of transmitted signals in linear modulation schemes typically undergoes significant distortion of both phase and amplitude when the modulated signals are boosted by a power amplifier for transmission to a receiver. The distortion is especially prevalent in transmitters that employ power efficient, but nonlinear, power amplifiers. As a result, linearization techniques have been developed to produce a desirable trade-off between a transmitter's efficiency and its linearity.
Among the more popular linearization techniques employable with transmitters is Cartesian feedback linearization. In this type of linearization technique, a Cartesian feedback section is provided after the power amplifier, which would otherwise introduce undesirable distortion into a modulated output signal. The Cartesian feedback section provides baseband in-phase and quadrature phase feedback signals that are demodulated from the output of the power amplifier. These signals are, by means of operational amplifiers, compared to in-phase and quadrature phase input signals of the power amplifier to provide a “predistortion” into the modulated signal prior to the power amplifier so that distortion introduced by the power amplifier is thereby offset achieving the desired linearization.
This predistortion essentially straightens or linearizes the nonlinear saturating transfer characteristic of the power amplifier and reduces the overall distortion. If a required power output level of the power amplifier is too large, however, the power amplifier may limit or clip its output signal thereby introducing a distortion level that Cartesian feedback linearization cannot fully correct. Unfortunately, the high operational efficiency of the power amplifiers usually necessitates an operational level that approaches the point of limiting or clipping, thereby presenting an important design and operational dilemma. Additionally, changes in an output load associated with the power amplifier may also cause changes in both loop gain and loop phase, which may lead to feedback loop instability including oscillation.
Accordingly, what is needed in the art is a way that effectively controls a power level associated with a transmitter employing Cartesian input and feedback signals to allow power amplifier efficiency while reducing distortion.